1. Field of Invention
The present invention relates, in general, to propeller shafts for vehicles, and, more particularly, to a propeller shaft for a vehicle, which is intended to sufficiently absorb a change in axial length owing to a low collapse load in the event of a vehicle crash and thereby to sufficiently absorb impact energy, thus being capable of reducing injury to passengers to the maximum.
2. Description of Related Art
Generally, as shown in FIG. 1, a propeller shaft 10 installed at a rear-wheel-drive vehicle is a power transmission device that smoothly transmits a driving force of a power train including an engine 1 and a transmission 2 to a rear axle 3. The propeller shaft 10 should have a sufficient torsional strength to allow for a smooth torque transfer, and should have a sufficient bending rigidity because the propeller shaft 10 is long in an axial direction.
Further, recently, safety regulations on crashes are considered as an important performance factor. Hence, in addition to the above-mentioned basic characteristics, there has been employed technology that reduces injury to a passenger by absorbing shocks through a change in axial length in the event of a vehicle crash.
However, the conventional propeller shaft 10 is problematic in that its change in axial length is not sufficient due to a high collapse load in the event of a vehicle crash, so that the propeller shaft 10 cannot sufficiently absorb shocks and thereby increases injury to a passenger.
That is, as shown in FIG. 2, the conventional propeller shaft 10 includes a front coupling 11 used to couple the propeller shaft 10 to the transmission 2, a front yoke 12 coupled to the front coupling 11, a front tube 13 coupled to the front yoke 12, a rear tube 15 connected to the front tube 13 via a universal joint 14, a rear yoke 16 coupled to the rear tube 15, and a rear coupling 17 used to couple the propeller shaft 10 to the rear axle 3.
Here, the front tube 13 is provided with a diameter change portion 13a that is made by a swaging process. The front tube 13 is divided into a smaller diameter portion 13b extending towards the front yoke 12 and a larger diameter portion 13c extending towards the universal joint 14, with respect to the diameter change portion 13a. 
The smaller diameter portion 13b is smaller in diameter than the larger diameter portion 13c, and the smaller diameter portion 13b and the larger diameter portion 13c are connected to each other via the diameter change portion 13a. 
Thus, if the impact energy generated in the event of a vehicle crash is transmitted through the power train to the propeller shaft 10, as shown in FIG. 3, the diameter change portion 13a is deformed while the smaller diameter portion 13b is inserted into the larger diameter portion 13c. Such a movement of the smaller diameter portion 13b allows the propeller shaft 10 to absorb the impact energy.
However, the conventional propeller shaft 10 is problematic in that a difference in diameter between the smaller diameter portion 13b and the larger diameter portion 13c is not large, so that a rearward moving amount of the smaller diameter portion 13b is small, and thus a change in axial length is not sufficient due to a high collapse load. Consequently, it is impossible to sufficiently absorb impact energy. In particular, injury to a passenger is increased due to the high deceleration of the vehicle.
Therefore, in order to increase a difference in diameter between the smaller diameter portion 13b and the larger diameter portion 13c, a method of either reducing the diameter of the smaller diameter portion 13b or increasing the diameter of the larger diameter portion 13c has been proposed. However, such a change in diameter changes a basic strength or raises a new problem due to weight and a package with peripheral components.
Further, in order to increase the rearward moving amount of the smaller diameter portion 13b, a method of changing the physical properties of a material and lowering the collapse load has been proposed. However, such a method is not desirable because it causes a reduction in basic strength of the propeller shaft 10.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.